FIGS. 42A and B show a structure of a conventional PTC heating element. The conventional PTC heating element includes a pair of comb-like electrodes (hereinafter as electrodes) 201, 202 and PTC resistor (hereinafter as resistor) 203 at a position where electric power is supplied therefrom on substrate 200. Substrate 200 is composed of a material having no flexibility at all or poor in the flexibility such as ceramics, insulated metal plate or polyester film. Electrodes 201, 202 are obtained by printing and drying a conductive paste. Resistor 203 is obtained by printing and drying a PTC composition ink (hereinafter as PTC ink). Substrate 200 and cover material 204 made of the same material as that of the substrate 200 cover and protect electrodes 201, 202 and PTC 203. FIG. 42A shows resistor 203 and cover material 204 while being partially cut-away.
In a case of using a polyester film as substrate 200 and cover material 204, a polyethylene hot melting resin, for example, is previously adhered to cover material 204. Substrate 200, electrodes 201, 202, resistor 203 and cover material 204 are bonded by way of the resin. The PTC resistor has been used as an over current protective device or a heating element small in the size and of a special shape while being fixed so as not to undergo mechanical stress such as bending for use in removing dews and frosts of automobile door mirrors and the like. With the practical point of view, terminals for power supply are necessary but are not illustrated.
As the PTC ink forming resistor 203, a base polymer including a crystalline polymer material, and a conductive material such as a carbon black, a metal powder or graphite are dispersed in a solvent. Such an ink is disclosed, for example, in Japanese Patent Unexamined Publication Nos. 56-13689, 6-96843 and 8-120182.
Resistor 203 has a characteristic that the resistance value increases along with temperature elevation and the resistance value increases abruptly when reaching a certain temperature to conduct self temperature control. Such a resistance-temperature characteristic is referred to as PTC (Positive Temperature Coefficient) characteristic and a resistor having such PTC characteristic is referred to as a PTC resistor. It is considered that the characteristic is developed by disconnection of a conductive path of the conductive material caused by cubical expansion of the crystalline polymer due to temperature elevation, which entails the increase of the resistance.
Since the conventional PTC heating element is formed on substrate 200 of poor flexibility as described above, it can not be used for application use as assembled in a car seat and fit to a human body, or can not be attached to an object of a curved surface such as a handle.
When a flexible resin film such as a resin or an elastomer is used for substrate 200, a PTC heating element having a temporal flexibility may be formed. When mechanical stress such as elongation exerts, however, disconnection or cracking may be caused to electrodes 201, 202 or resistor 203. Development of the PTC characteristics is caused by the change of the chained state of the conductive material caused by the thermal change in volume of the crystalline polymer. Accordingly, it may be considered easily that physical or mechanical change in dimension of the substrate gives a significant effect on the characteristic of the PTC resistor. Therefore, a PTC heating element capable of enduring the use under a practical circumstance where repetitive bending load is applied has not yet been developed.